We propose to determine the structure of HIV-1 VLPs halted at selected stages throughout maturation by cryo-electron tomography. This method will enable us to determine (1) the morphological changes the virus undergoes during maturation, (2) the structure of immature and mature HIV-1 VLPs with the Envelope protein (Env) expressed and how it is arranged in the matrix (MA) layer, and (3) the positions of other viral components within the particles. To achieve these aims, in collaboration with researchers at the University of Utah, we plan to (1) purify HIV-1 mutants halted at several 'developmental' stages, (2) image the HIV-1 mutants by cryo-electron tomography and calculate the structures through computational methods, and (3) determine the structural variability present in the specific viral proteins during maturation. Our studies will provide the highest resolution structures of whole, intact frozen-hydrated HIV-1 VLPs from which other researchers may determine the placement of other proteins and data from which new therapies may be designed to combat or eradicate HIV-1 infection.